Size-dependent sex allocation and the expression of andromonoecy in a protogynous perennial herb: both size and timing matter

The optimal life history and sex allocation of perennial hermaphrodites should depend on both their size and the relative costs and benefits of reproducing through male versus female functions. Theory predicts that insect-pollinated perennials should increase their allocation to female function with size, while the mating environment hypothesis predicts that allocation to male function should track mating opportunities over the course of flowering. We test these two predictions by inferring male and female reproductive success in the protogynous perennial herb Pulsatilla alpina by tracking the patterns and dynamics of sex allocation over time for marked individuals over a range of sizes. We found that small individuals tend to produce only male flowers and that both small and larger individuals produced male flowers at the beginning of the flowering season when mating opportunities were high. By considering within-population variation in life history and phenology jointly rather than separately, and by considering both the tradeoff costs and benefits of allocation to male versus female functions, our results provide new insights into the evolution of both gender diphasy and andromonoecy in perennial plants that are constrained by a dichogamous flowering strategy.